1. Field of the Invention
This invention relates to semiconductor controlled rectifiers and a method of making the same.
2. Description of the Prior Art
A semiconductor controlled rectifier (SCR) may be produced by several different P-N junction-forming techniques. Suitable techniques include alloying, diffusion and epitaxial growth. Diffusion and alloying have proved to be the most popular techniques because they lend themselves to the economical mass production of SCRs. However, both techniques result in P-N junctions which are not very sharply defined although they may meet the requirement of a "step" P-N junction. Also, the width of the P-N junction varies with the process temperature employed. In addition, the diffusion process results in a region having non-uniform impurity concentration throughout the region.
The material of regions formed by alloying techniques is recrystallized material. These regions are limited in depth and area covered because the alloying process requires the formation of a molten mixture of the semiconductor material and either a donor or an acceptor material as the impurity material and subsequent solidification of the same to form the desired region. Moreover, it is very difficult to control the exact depth of an alloyed junction because the amount of molten mixture that is formed on alloying is a function of both the absolute temperature and the amount of metal-rich component placed on the semiconductor to form the alloy. In addition, the molten material must be prevented from flowing outside of a confined area otherwise the device may be inoperable. Usually, the process is limited to that portion of the device immediately adjacent to an electrical contact. Thus, most often, the alloying process is practiced when an electrical contact is affixed to the device.
In either case, the diffusion and alloying techniques are practiced only through a surface area and form regions therein which are only adjacent to the major surfaces involved. Because of the elapsed time and high temperature required for the diffusion process, the regions produced are limited in size. Any additional high temperature processing of the processed body readily affects the P-N junctions which have been previously formed and tends to enlarge the different regions and alter the configuration of the P-N junction.
Epitaxial growth techniques overcome some of the difficulties encountered by the diffusion and alloying techniques. Regions of fairly uniform resistivity are easily grown. However, the temperatures and the rate of growth of material are factors which limit the thickness of a region that can be grown economically. The P-N junction which results from epitaxial growth techniques is more sharply defined than the junctions made by alloying and diffusion.
In all instances, however, theoretical breakdown voltages and operating characteristics are not achieved. High temperatures for extended periods of time adversely affect the crystalline perfection of the substrate material. It also causes the P-N junction already present in the processed material to move and may also result in the out diffusion of dopant impurities thereby changing the resistivity of the region or regions of semiconductor material involved. Additionally, the processes deposit impurities within or on surfaces of the regions and at P-N junctions formed by the epitaxial process thereby detrimentally affecting the operating characteristics of the devices.
None of the semiconductor controlled rectifiers manufactured to date have several regions made from recrystallized substrate material. In addition, the semiconductor controlled rectifiers have usually been limited to four regions for controlled rectifier applications and to five regions for bidirectional switching applications. Such structural limitations have resulted from extensive process time and high process temperatures encountered in making the devices.
An object of this invention is to provide a new and improved semiconductor controlled rectifier.
Another object of this invention is to provide a new and improved semiconductor controlled rectifier wherein selected alternate regions of opposite type conductivity having a constant uniform dopant impurity concentration throughout the selected regions are formed from recrystallized regions of the substrate.
Another object of this invention is to provide a new and improved semiconductor controlled rectifier wherein the P-N junctions formed therein are well defined and have a minimum width to provide a step P-N junction.
Other objects of this invention will, in part, be obvious and will, in part, appear hereinafter.